shadow/man/passwd.1.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
   SPDX-FileCopyrightText: 1989 - 1994, Julianne Frances Haugh
   SPDX-FileCopyrightText: 2007 - 2011, Nicolas François
   SPDX-License-Identifier: BSD-3-Clause
-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY ENCRYPT_METHOD        SYSTEM "login.defs.d/ENCRYPT_METHOD.xml">
<!ENTITY MD5_CRYPT_ENAB        SYSTEM "login.defs.d/MD5_CRYPT_ENAB.xml">
<!ENTITY OBSCURE_CHECKS_ENAB   SYSTEM "login.defs.d/OBSCURE_CHECKS_ENAB.xml">
<!ENTITY PASS_ALWAYS_WARN      SYSTEM "login.defs.d/PASS_ALWAYS_WARN.xml">
<!ENTITY PASS_CHANGE_TRIES     SYSTEM "login.defs.d/PASS_CHANGE_TRIES.xml">
<!ENTITY PASS_MAX_LEN          SYSTEM "login.defs.d/PASS_MAX_LEN.xml">
<!ENTITY SHA_CRYPT_MIN_ROUNDS  SYSTEM "login.defs.d/SHA_CRYPT_MIN_ROUNDS.xml">
<!-- SHADOW-CONFIG-HERE -->
]>
<refentry id='passwd.1'>
  <!--  $Id$ -->
  <refentryinfo>
    <author>
      <firstname>Julianne Frances</firstname>
      <surname>Haugh</surname>
      <contrib>Creation, 1989</contrib>
    </author>
    <author>
      <firstname>Thomas</firstname>
      <surname>Kłoczko</surname>
      <email>kloczek@pld.org.pl</email>
      <contrib>shadow-utils maintainer, 2000 - 2007</contrib>
    </author>
    <author>
      <firstname>Nicolas</firstname>
      <surname>François</surname>
      <email>nicolas.francois@centraliens.net</email>
      <contrib>shadow-utils maintainer, 2007 - now</contrib>
    </author>
  </refentryinfo>
  <refmeta>
    <refentrytitle>passwd</refentrytitle>
    <manvolnum>1</manvolnum>
    <refmiscinfo class="sectdesc">User Commands</refmiscinfo>
    <refmiscinfo class="source">shadow-utils</refmiscinfo>
    <refmiscinfo class="version">&SHADOW_UTILS_VERSION;</refmiscinfo>
  </refmeta>
  <refnamediv id='name'>
    <refname>passwd</refname>
    <refpurpose>change user password</refpurpose>
  </refnamediv>

  <refsynopsisdiv id='synopsis'>
    <cmdsynopsis>
      <command>passwd</command>
      <arg choice='opt'>
	<replaceable>options</replaceable>
      </arg>
      <arg choice='opt'>
        <replaceable>LOGIN</replaceable>
      </arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1 id='description'>
    <title>DESCRIPTION</title>
    <para>
      The <command>passwd</command> command changes passwords for user accounts.
      A normal user may only change the password for their own account, while
      the superuser may change the password for any account.
      <command>passwd</command> also changes the account or associated
      password validity period.
    </para>

    <refsect2 id='password_changes'>
      <title>Password Changes</title>
      <para>
	The user is first prompted for their old password, if one is
	present. This password is then encrypted and compared against the
	stored password. The user has only one chance to enter the correct
	password. The superuser is permitted to bypass this step so that
	forgotten passwords may be changed.
      </para>

      <para>
	After the password has been entered, password aging information is
	checked to see if the user is permitted to change the password at
	this time. If not, <command>passwd</command> refuses to change the
	password and exits.
      </para>

      <para>
	The user is then prompted twice for a replacement password. The
	second entry is compared against the first and both are required to
	match in order for the password to be changed.
      </para>

      <para>
	Then, the password is tested for complexity.
	<command>passwd</command> will reject any password which is not
	suitably complex.  Care must be taken not to include the system
	default erase or kill characters.
      </para>

    </refsect2>

    <refsect2 id='hints_for_user_passwords'>
      <title>Hints for user passwords</title>
      <para>
	The security of a password depends upon the strength of the
	encryption algorithm and the size of the key space. The legacy
	<emphasis>UNIX</emphasis> System encryption method is based on the
	NBS DES algorithm. More recent methods are now recommended (see
	<option>ENCRYPT_METHOD</option>). The size of the key space
	depends upon the randomness of the password which is selected.
      </para>

      <para>
	Compromises in password security normally result from careless
	password selection or handling. For this reason, you should not
	select a password which appears in a dictionary or which must be
	written down. The password should also not be a proper name, your
	license number, birth date, or street address. Any of these may be
	used as guesses to violate system security.
      </para>

      <para>
	As a general guideline, passwords should be long and random.  It's
	fine to use simple character sets, such as passwords consisting
	only of lowercase letters, if that helps memorizing longer
	passwords.  For a password consisting only of lowercase English
	letters randomly chosen, and a length of 32, there are 26^32
	(approximately 2^150) different possible combinations.  Being an
	exponential equation, it's apparent that the exponent (the length)
	is more important than the base (the size of the character set).
      </para>

      <para>
	You can find advice on how to choose a strong password on
	http://en.wikipedia.org/wiki/Password_strength
      </para>
    </refsect2>
  </refsect1>

  <refsect1 id='options'>
    <title>OPTIONS</title>
    <para>
      The options which apply to the <command>passwd</command> command are:
    </para>
    <variablelist remap='IP'>
      <varlistentry>
	<term>
	  <option>-a</option>, <option>--all</option>
	</term>
	<listitem>
	  <para>
	    This option can be used only with <option>-S</option> and causes show
	    status for all users.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-d</option>, <option>--delete</option>
	</term>
	<listitem>
	  <para>
	    Delete a user's password (make it empty). This is a quick way
	    to disable a password for an account. It will set the named
	    account passwordless.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-e</option>, <option>--expire</option>
	</term>
	<listitem>
	  <para>
	    Immediately expire an account's password. This in effect can
	    force a user to change their password at the user's next login.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><option>-h</option>, <option>--help</option></term>
	<listitem>
	  <para>Display help message and exit.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-i</option>, <option>--inactive</option>&nbsp;<replaceable>INACTIVE</replaceable>
	</term>
	<listitem>
	  <para>
	    This option is used to disable an account after the password has
	    been expired for a number of days. After a user account has had
	    an expired password for <replaceable>INACTIVE</replaceable>
	    days, the user may no longer sign on to the account.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-k</option>, <option>--keep-tokens</option>
	</term>
	<listitem>
	  <para>
	    Indicate password change should be performed only for expired
	    authentication tokens (passwords). The user wishes to keep their
	    non-expired tokens as before.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-l</option>, <option>--lock</option>
	</term>
	<listitem>
	  <para>
	    Lock the password of the named account. This option disables a
	    password by changing it to a value which matches no possible
	    encrypted value (it adds a ´!´ at the beginning of the
	    password).
	  </para>
	  <para>
	    Note that this does not disable the account. The user may
	    still be able to login using another authentication token
	    (e.g. an SSH key). To disable the account, administrators
	    should use <command>usermod --expiredate 1</command> (this set
	    the account's expire date to Jan 2, 1970).
	  </para>
	  <para>
	    Users with a locked password are not allowed to change their
	    password.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-n</option>, <option>--mindays</option>&nbsp;<replaceable>MIN_DAYS</replaceable>
	</term>
	<listitem>
	  <para>
	    Set the minimum number of days between password changes to
	    <replaceable>MIN_DAYS</replaceable>. A value of zero for this field
	    indicates that the user may change their password at any time.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-q</option>, <option>--quiet</option>
	</term>
	<listitem>
	  <para>
	    Quiet mode.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-r</option>, <option>--repository</option>&nbsp;<replaceable>REPOSITORY</replaceable>
	</term>
	<listitem>
	  <para>
	    change password in <replaceable>REPOSITORY</replaceable> repository
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-R</option>, <option>--root</option>&nbsp;<replaceable>CHROOT_DIR</replaceable>
	</term>
	<listitem>
	  <para>
	    Apply changes in the <replaceable>CHROOT_DIR</replaceable>
	    directory and use the configuration files from the
	    <replaceable>CHROOT_DIR</replaceable> directory.
	    Only absolute paths are supported.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-S</option>, <option>--status</option>
	</term>
	<listitem>
	  <para>
	    Display account status information. The status information
	    consists of 7 fields. The first field is the user's login name. 
	    The second field indicates if the user account has a locked
	    password (L),
	    has no password (NP), or has a usable password (P). The third
	    field gives the date of the last password change. The next four
	    fields are the minimum age, maximum age, warning period, and
	    inactivity period for the password. These ages are expressed in
	    days.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-u</option>, <option>--unlock</option>
	</term>
	<listitem>
	  <para>
	    Unlock the password of the named account. This option
	    re-enables a password by changing the password back to its
	    previous value (to the value before using the
	    <option>-l</option> option).
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-w</option>, <option>--warndays</option>&nbsp;<replaceable>WARN_DAYS</replaceable>
	</term>
	<listitem>
	  <para>
	    Set the number of days of warning before a password change is
	    required. The <replaceable>WARN_DAYS</replaceable> option is
	    the number of days prior to the password expiring that a user
	    will be warned that their password is about to expire.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-x</option>, <option>--maxdays</option>&nbsp;<replaceable>MAX_DAYS</replaceable>
	</term>
	<listitem>
	  <para>
	    Set the maximum number of days a password remains valid. After
	    <replaceable>MAX_DAYS</replaceable>, the password is required
	    to be changed.
	  </para>
	  <para>
	    Passing the number <emphasis remap='I'>-1</emphasis> as
	    <replaceable>MAX_DAYS</replaceable> will remove checking a
	    password's validity.
	  </para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id='caveats'>
    <title>CAVEATS</title>
    <para>
      Password complexity checking may
      vary from site to site. The user is urged to select a password as
      complex as he or she feels comfortable with.
    </para>
    <para>
      Users may not be able to 
      change their password on a system if NIS is enabled and they are not 
      logged into the NIS server.
    </para>
    <para condition="pam">
      <command>passwd</command> uses PAM to authenticate users and to
      change their passwords.
    </para>
  </refsect1>

  <refsect1 id='configuration' condition="no_pam">
    <title>CONFIGURATION</title>
    <para>
      The following configuration variables in
      <filename>/etc/login.defs</filename> change the behavior of this
      tool:
    </para>
    <variablelist>
      &ENCRYPT_METHOD;
      &MD5_CRYPT_ENAB;
      &OBSCURE_CHECKS_ENAB;
      &PASS_ALWAYS_WARN;
      &PASS_CHANGE_TRIES;
      &PASS_MAX_LEN; <!-- documents also PASS_MIN_LEN -->
      &SHA_CRYPT_MIN_ROUNDS;
    </variablelist>
  </refsect1>

  <refsect1 id='files'>
    <title>FILES</title>
    <variablelist>
      <varlistentry>
	<term><filename>/etc/passwd</filename></term>
	<listitem>
	  <para>User account information.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/shadow</filename></term>
	<listitem>
	  <para>Secure user account information.</para>
	</listitem>
      </varlistentry>
      <varlistentry condition="no_pam">
	<term><filename>/etc/login.defs</filename></term>
	<listitem>
	  <para>Shadow password suite configuration.</para>
	</listitem>
      </varlistentry>
      <varlistentry condition="pam">
	<term><filename>/etc/pam.d/passwd</filename></term>
	<listitem>
	  <para>PAM configuration for <command>passwd</command>.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id='exit_values'>
    <title>EXIT VALUES</title>
    <para>
      The <command>passwd</command> command exits with the following values:
      <variablelist>
	<varlistentry>
	  <term><replaceable>0</replaceable></term>
	  <listitem>
	    <para>success</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><replaceable>1</replaceable></term>
	  <listitem>
	    <para>permission denied</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><replaceable>2</replaceable></term>
	  <listitem>
	    <para>invalid combination of options</para>
	    </listitem>
	</varlistentry>
	<varlistentry>
	  <term><replaceable>3</replaceable></term>
	  <listitem>
	    <para>unexpected failure, nothing done</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><replaceable>4</replaceable></term>
	  <listitem>
	    <para>unexpected failure, <filename>passwd</filename> file missing</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><replaceable>5</replaceable></term>
	  <listitem>
	    <para><filename>passwd</filename> file busy, try again</para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term><replaceable>6</replaceable></term>
	  <listitem>
	    <para>invalid argument to option</para>
	  </listitem>
	</varlistentry>
      </variablelist>
    </para>
  </refsect1>

  <refsect1 id='see_also'>
    <title>SEE ALSO</title>
    <para>
      <citerefentry>
	<refentrytitle>chpasswd</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>makepasswd</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>passwd</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>shadow</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <phrase condition="no_pam">
	<citerefentry>
	  <refentrytitle>login.defs</refentrytitle><manvolnum>5</manvolnum>
	</citerefentry>,
      </phrase>
      <citerefentry>
	<refentrytitle>usermod</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>.
    </para>

    <para>
	The following web page comically (yet correctly) compares the
	strength of two different methods for choosing a password:
	"https://xkcd.com/936/"
    </para>
  </refsect1>
</refentry>